Make inserting links in user notes more user friendly

Description

To make inserting links in user notes more user friendly the link Quicktag button should be added to the note editors.

The following patch adds the link button. The search panel and checkbox to open links in a new tab in the modal window are hidden with CSS. I've also disabled the search query even though the search panel is hidden.

No, I've hidden it with CSS and for good measure disabled the query as well. The query is expensive and the order of the results is different than the normal search results. It felt cluttered. But as you say, it could still be useful.

To have the same results (order) as a normal search we could set suppress_filters to false, but that would make the query even more expensive.

I've also just noticed we should restrict the post type to reference or handbook posts.